[MINOR] fix: Change to get coordinator appCurrent value from metrics (a…

…pache#2129)

### What changes were proposed in this pull request?

Change to get coordinator appCurrent value from metrics

### Why are the changes needed?

Keep consistent with `appTotality`, and in case of incorrect result returned from `getAppIds`.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

No need.

coordinator/src/main/java/org/apache/uniffle/coordinator/web/resource/ApplicationResource.java

@@ -51,7 +51,7 @@ public Response<Map<String, Integer>> getAppTotality() {
  return execute(
  () -> {
  Map<String, Integer> appTotalityMap = Maps.newHashMap();
- appTotalityMap.put("appCurrent", getApplicationManager().getAppIds().size());
+ appTotalityMap.put("appCurrent", (int) CoordinatorMetrics.gaugeRunningAppNum.get());
  appTotalityMap.put("appTotality", (int) CoordinatorMetrics.counterTotalAppNum.get());
  return appTotalityMap;
  });